The Dissolution of Vanadium Pentoxide in Aqueous Solutions of Oxalic and Mineral Acids.

The dissolution of vanadium pentoxide in perchloric and oxalic acids has been studied at 25.0 degreesC and ionic strength 0.50 mol dm-3 (NaClO4). The results are described in terms of an extended surface complexation approach. The rate data in HClO4 are used to derive an estimated value of the pH of the point of zero charge, pHo approximately 3. The whole set of rate data is accounted for by surface protolytic and complexation reactions, followed by the first order transfer of surface species to solution. The transfer rates are fast but are controlled by surface reactions; the first order rate constants are of the order of 1 s-1. Protonated, mono- and dioxalato surface complexes are involved. Because of the fast transfer rates, it is possible that the surface concentrations are steady state rather than equilibrium values. Thus, the idea of surface complexes as transient species is put forward. Because of the pH dependence of surface protonation and complexation by oxalate, this scheme leads to dissolution rate enhancements by oxalic acid (with respect to dissolution in mineral acid) that increase with pH. Copyright 1999 Academic Press.